.\"	$OpenBSD: listen.2,v 1.14 2014/12/10 19:19:00 schwarze Exp $
.\"	$NetBSD: listen.2,v 1.7 1996/02/16 20:38:45 phil Exp $
.\"
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.\"     @(#)listen.2	8.2 (Berkeley) 12/11/93
.\"
.Dd $Mdocdate: December 10 2014 $
.Dt LISTEN 2
.Os
.Sh NAME
.Nm listen
.Nd listen for connections on a socket
.Sh SYNOPSIS
.Fd #include <sys/socket.h>
.Ft int
.Fn listen "int s" "int backlog"
.Sh DESCRIPTION
To accept connections, a socket
is first created with
.Xr socket 2 ,
a willingness to accept incoming connections and
a queue limit for incoming connections are specified with
.Fn listen ,
and then the connections are
accepted with
.Xr accept 2 .
The
.Fn listen
call applies only to sockets of type
.Dv SOCK_STREAM
or
.Dv SOCK_SEQPACKET .
.Pp
The
.Fa backlog
parameter defines the maximum length the queue of
pending connections may grow to.
If a connection
request arrives with the queue full the client may
receive an error with an indication of
.Er ECONNREFUSED ,
or, if the underlying protocol supports retransmission,
the request may be ignored so that retries may succeed.
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
.Fn listen
will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
The argument
.Fa s
is not a valid descriptor.
.It Bq Er ENOTSOCK
The argument
.Fa s
is not a socket.
.It Bq Er EOPNOTSUPP
The socket is not of a type that supports the operation
.Fn listen .
.It Bq Er EINVAL
The socket is already connected.
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr connect 2 ,
.Xr socket 2 ,
.Xr sysctl 8
.Sh STANDARDS
The
.Fn listen
function conforms to
.St -p1003.1-2008 .
.Sh HISTORY
The
.Fn listen
system call first appeared in
.Bx 4.1c .
.Sh BUGS
The
.Fa backlog
is currently limited (silently) to the value of the kern.somaxconn
sysctl, which defaults to 128.
